1. Field of the Invention
The present invention relates to a method of manufacturing a solid preform by moving a heat source parallel to the longitudinal axis of a substrate tube, whose inner surface is coated with one or more doped or undoped glass layers, so as to collapse the substrate tube into the solid preform in a number of passes, with an etchant being supplied to the interior of the substrate tube after a number of passes of the heat source.
2. Description of the Related Art
In U.S. Pat. No. 4,793,863, a gaseous etchant is passed through the interior of the substrate tube during the collapsing of the substrate tube for forming the solid preform, which gaseous etchant consists of oxygen and 5–30 vol. % of C2F6. During the collapsing of the substrate tubes, which are internally coated with glass layers doped with a relatively volatile dopant, for example germanium dioxide, a part of the dopant may evaporate from the glass layers, which results in a disturbance in the refractive index profile. In such a situation, the diffusion of one or more dopants from the glass layers situated further away from the centre will also play a part. Consequently, the profile disturbance has an adverse influence on the optical characteristics of the fibre.
From European patent application No. 1 035 083 there is known a method of preventing such an undesirable deviation in the refractive index profile in the centre of the core, which method comprises the collapsing of the substrate tube in two steps, viz. heating the substrate tube at a first collapsing temperature, for the purpose of decreasing the dimension of the central duct of the substrate tube, flowing an etchant gas through the central duct at a temperature about 200–400° C. lower than the minimum collapsing temperature, for the purpose of etching away part of the core from the central duct of the substrate tube, and subsequently fully collapsing the substrate tube at a second collapsing temperature, which second collapsing temperature is higher than the first collapsing temperature. According to the embodiments described in said document, the inner diameter of the substrate tube during the etching is 5 mm (embodiment 1), 1.5 mm (embodiment 2), and 3 mm (embodiment 3). Said document does not mention the fact that the moment of supplying etchant is critical, however, let alone that the inner diameter of the substrate tube must range within predetermined values along the length thereof.
From U.S. Pat. No. 6,131,413 there are known two embodiments of a method of collapsing a substrate tube, which embodiments are to prevent variations in the core diameter along the longitudinal axis of the final solid preform. According to a first embodiment, a certain value of the pressure at the end of the preform is determined, after which the speed of movement of the heat source is controlled as a function of said pressure value. According to a second embodiment, the flow rate of the etchant is controlled in dependence on the pressure measurement. Further details with regard to the moment of supplying the etchant, in particular with regard to the dimension of the inner diameter of the substrate tube during the etching process are not known from said document.
According to European patent application No. 0 972 752 an optical preform is manufactured according to the rod-in-rod principle. Although said document relates to the manufacture of optical preforms for about 400 km of fibre, using an MCVD process with etching of the interior of the coated substrate tube prior to collapsing, this document does not provide any information with regard to the inner diameter of the substrate tube at the moment of supplying the etchant.